Expose more information to the API (playlist folders, escaped location)

Hi Liam, I like to contribute some minor extensions.

Currently, the Playlist class doesn't contain information, whether or not a playlist is actually a folder for other playlists. But I want to skip folders. Hence, I need to distinguish between playlists and folders. Moreover, it would be nice for a client to know which playlist belongs to which folder. iTunes' XML already contains these information ("folder", "playlist_persistent_id", "parent_persistent_id"), we only have to expose them to the API.
Currently, the "location" property of Song contains the unescaped path, which is fine for the most use cases. However, I actually need the escaped location (I like to write the paths to Rhythmbox playlists). So I have to escape the location which is error-prone due to unclear safe characters (a lot of try & error until I finally found the characters that should not be escaped). Fortunately, the iTunes XML already contains the escaped path (which also matches Rhythmbox' format), we only need to expose it in the API.

Are you okay with that? Looking forward to your response! Cheers, Philipp

liamks / libpytunes